I think that Shimano SL-BS78 shifters can be used in friction mode.

10 speed shifters; they won't pull enough total cable for 11 speed.

Same story for Microshift. 11-speed = index only.

It might be possible to disassemble and remove the detente mechanism, but you're probably in your own there.

Alternately take a 10-speed shifter and bodge a travel agent in there, but again you're on your own.

OP: What's the reason?

Dia compe do a friction shifter that should work.

 I have an old trainer bike with a shit ton of miles on it. I take bad care of it. It shifts perfectly for the last few years, probably 10,000 miles. I never even adjusted the deraileur when replacing chains.

It is 10spd friction. Thinking about that for my next tt bike.