Deore FC-M615 single 64 BCD chainring I want to build a 1x9 drivetrain MTB so I'm wondering if I can set this FC-M615 crankset to a single 64 BCD 28T chainring, from what I can see the XT M8000 is compatible and allows 11-speed cassette but I can't find any more information.

Why 28T? because it is what I felt more comfortable with, I have made the gear calculation on this site

Regarding the initial question, is that possible? will it work properly?
You need an offset chainring for this to work well. Otherwise the front chainline will wind up too far in, and when you're riding it with the small cog the chain angle will be extreme and likely problematic, i.e. as though you were crosschaining a double. There's a Wolf Tooth 28t x 64bcd ring that does this. (I'm assuming your frame is either 135 or 142x12; if it's Boost there's not a way for this to work reasonably.)

Presuming you're suggesting using the small ring off a double FC-M8000, that won't work because it's just a normal ring for derailleur use. In addition to not being offset and thereby putting the chainline too far in, it will want to cause rather than prevent derailment in the smaller back cogs when used without an FD in the way. There is zero reason to not use a proper NW ring.

Exactly how bad the results are if you go with a chainline that's too far in also depends on chainstay length and frame alignment.