Legally the stations will still be required to use the full set of call letters for TOH ID. There is no requirement to use government-imposed call letters as a station's marketing brand. That's what's at issue here. As others have pointed out, lots of stations have adopted other brands that aren't strictly the call letters.
This is nothing new. What I find different is the company is saying they are "digital-first." I'm surprised there haven't been any comments about that.
https://www.pbs.org/digital-studios/
I don't think that's shocking at this point though for local media outlets to emphasize digital first given that PBS affiliates like KQED and others have been doing shows for PBS Digital Studios for some time though.