Ok, I think I figured this out. Starting from 2020, for mixed-user rentals, TT lets you propagate Interest/Taxes/Other expenses in SchE column C through Sch1 to 1040. Previous versions didn't propagate anything. "Allocated to personal use" portion still goes through SchA.
The obvious benefit of this is that it can mitigate the 10k SALT limit. If you have property taxes > 10k, then by renting the place part of the year while still using it most of the year you can route portion of those taxes through Sch1 as businesses losses. When I tried TT18-19 didn't let me do that.
TT20+ still don't let you expense other categories (e.g. Supplies/Repairs/Utilities/etc.) for mixed-use rental property, so it's only Interest/Taxes (which would have gone to SchA otherwise) and Other expenses (that you are responsible to make sure are truly deductible). Since other comes from Miscellaneous expenses in UI, that's where I think TT could do better - it's not obvious which of those expenses are acceptable for mixed-use rental active income deduction and which aren't, especially for folks w/MAGI>150k.
Mystery solved.
Cheers, MM
