The NFL has used a limited flex schedule for several years, with the ability to change a Sunday day game to Sunday evening. However those games are usually (always?) announced with the initially scheduled time rather than TBD.
Week 15 of 2021 is after the college regular season ends and is the first week when NFL Saturday games are played. The 5 TBD games can be flexed over to Saturday and I think they didn't want to announce a tentative time and have it change to a different day.
The deadline for scheduling is 4 weeks prior to the game (which by my calendar is today), so this should be announced pretty quickly.
I just noticed the NFL Week 15 Schedule page has a blurb about this week (and about the similar week 18).
2021 TBD Games
In Week 15, two of five designated matchups will be played on Saturday
with the remainder to be played on Sunday. Specific dates and start
times for such designated Week 15 matchups will be determined and
announced no later than four weeks prior to game day.
ETA, seems to be later than the "four weeks prior to game day" deadline, but all games were scheduled and TBDs for Week 15 removed from the NFL schedule page on November 23.