I think I saw absolute black doing a 28t for a 64 bcd chain set (conventional as it gets).
Blackspire Do a 26t and a 28t for an 80bcd crank (SRAM and raceface).
I run a 30t with an 11-36 on my 29″ hardtail for local hooning. Not the hilliest in Lancs tho. Having said that fancy trying a 40t rear cog to extend the life of my knees and as my fitness seems to drop off as the kids grow and take over my spare time.
My 29″ am bike has a 28t cog with 10-42 cassette but that tends to get used in the lakes on the steep stuff. With the 10t cog I can go as fast as I need but the 28-42 combo gets me up most things I have the legs for.