You can see "splash guards" used by (almost) everyone in this video of a German tracing center, but (almost) nobody wears masks there. (Well, one spokesperson did wear mask and no shield.) So are there any studies on the effectiveness of such a (shield only) an approach at preventing respiratory illness (not necessarily COVID-19) transmission?
To partly answer my own question, after trying quite a few different potential keywords for these shields, I did find one 2014 paper
Efficacy of face shields against cough aerosol droplets from a cough simulator
Health care workers are exposed to potentially infectious airborne particles while providing routine care to coughing patients. However, much is not understood about the behavior of these aerosols and the risks they pose. We used a coughing patient simulator and a breathing worker simulator to investigate the exposure of health care workers to cough aerosol droplets, and to examine the efficacy of face shields in reducing this exposure. Our results showed that 0.9% of the initial burst of aerosol from a cough can be inhaled by a worker 46 cm (18 inches) from the patient. During testing of an influenza-laden cough aerosol with a volume median diameter (VMD) of 8.5 μm, wearing a face shield reduced the inhalational exposure of the worker by 96% in the period immediately after a cough. The face shield also reduced the surface contamination of a respirator by 97%. When a smaller cough aerosol was used (VMD = 3.4 μm), the face shield was less effective, blocking only 68% of the cough and 76% of the surface contamination. In the period from 1 to 30 minutes after a cough, during which the aerosol had dispersed throughout the room and larger particles had settled, the face shield reduced aerosol inhalation by only 23%. Increasing the distance between the patient and worker to 183 cm (72 inches) reduced the exposure to influenza that occurred immediately after a cough by 92%. Our results show that health care workers can inhale infectious airborne particles while treating a coughing patient. Face shields can substantially reduce the short-term exposure of health care workers to large infectious aerosol particles, but smaller particles can remain airborne longer and flow around the face shield more easily to be inhaled. Thus, face shields provide a useful adjunct to respiratory protection for workers caring for patients with respiratory infections. However, they cannot be used as a substitute for respiratory protection when it is needed.
So I guess the question (now) is what studies did those German tracing centers managers read. (I mean do they allow people who are coughing to continue to work... because otherwise, just based on that study, this method doesn't seem very effective by itself. And even then, the aerosols generated by cough still find their way around shields at some distances, albeit in reduced proportion.)
And a 2016 review citing that paper for much of the concrete evidence also says:
as highlighted in a recent Institute of Medicine report, little is known about the effectiveness of face shields in preventing the transmission of viral respiratory diseases.
(Much of the [other] research cited in that review focuses on comparing shields with safety glasses, so not directly useful for my answering my question.)
But citing a (much older) paper that has some relevance:
Utilizing an aerosolized dye (mean particle size 4.8 µm) emitted at a distance of 6 in (15 cm) from subjects wearing two models of face shields, Christensen et al.  noted that the face shields were inferior to two models of surgical face masks tested similarly for particle penetration and that the combination of one of the facemasks with a face shield improved results only marginally. These face shield results were attributable to the lack of a peripheral fit.
However, citing a 1995 paper:
Use of face shields alone for three months, compared with the use of face masks alone for an equal period, during thoracic and general surgeries resulted in no difference in infection rates of patients. Clearly, there is a need for further research into the protection from infectious airborne pathogens afforded by face shields either worn alone or in conjunction with other PPE worn simultaneously.
So I guess there is some data that under some conditions face shields are equivalent with respirators. However, in the conclusions section, that review doesn't see that data as very convincing for a general recommendation and instead says (citing a bit too many guidelines so I won't link them individually):
Use of a face shield alone for eye, face, and mucous membrane protection from contamination by body fluids is likely insufficient and it has been recommended that in those situations where a face shield is used to protect against splash or splatter, a medical/surgical mask would also be indicated. Face shields are not meant to function as primary respiratory protection and should not be used alone because aerosols can flow behind the visor,[16, 19, 21, 41] so a protective facemask (medical/surgical mask, N95 FFR, etc.) should be worn concurrently. In those instances where aerosolization of body fluids of infectious individuals is likely to occur (suctioning the airway, intubation, etc.), a respirator (e.g., N95 FFR, at a minimum) should be used in conjunction with the face shield.
So I think the German tracing center practice looks pretty questionable in terms of effectiveness...