Just wondering about this, haven't come across a situation lately where I kept K&T, but could/should it be AFCI/GFCI'ed at the breaker?

Anyone have an opinion on that?

Thats a good question, don't know , never tried it. IMO, it might not work due to the fact that polaritys are always crossed at times in K&T wiring,and also switched neutrals, encountered in old homes, and hot 3 ways.

Arc fault breakers should still work with K and T.
Ground fault breakers might not work if the neutral and hot polarity is switched. Best to use GFCI receptacles, after determining the correct polarity.

Also be forewarned, a lot of the old metal boxes aren't large enough to accommodate a GFCI receptacle.

When I change a panel and there is K&T I put a GFI switch on the K&T, by the panel. The only problem is that sometimes the circuits share a neutral, or two.

When I add a bedroom plug, I put the K&T for the bedroom on the AFCI with the new plug. Same problem with shared neutrals.

Haven't had any problems with polarity.