Flamestrike
Legend
I wouldn't be surprised if there is some sage advice or errata on it. The only reason the feats from Xanathar's say "race from phb" is because the book is old and wasn't future proofed for this situation. Pretty much just need to remove the mention of phb so that the people who are needlessly strict on this matter can't point to it.
Look at Squat Nimbleness that only requires 'Dwarf or a Small race'. I would have no issue allowing Goblins or similar small races, not found in the PHB (including C-Lineage small races) to take that feat.
There is just a lot of handwringing about custom lineages from a mechanical perspective that I just dont see. If those concerns are players 'gaming the rules' to create an 'OP or superior build' there is a solution for that (the DM). Presuming the rules are used in the spirit they're intended, then I honestly dont see anything mechanically ZOMG about allowing custom lineages, nor do I see anything ZOMG about allowing them access to racial feats from XGTE (presuming they have a reasonable tie to the relevant lineage).
I've always allowed PCs to re-fluff or re-skin races in any event (an Aaracrocka Monk is one example - re-skinned as a Human - because the PC wanted to do the whole Wuxia Human flying Monk thing, but didn't want to look like a bird man).
It seems the handwringing is due to one of three things (when Clineage is compared to Vuman):
1) Darkvision in exchange for a skill Proficiency in whatever you want
2) Non Humans getting access to Racial feats 3 levels earlier (and those levels go fast) in exchange for losing a ton of racial abilities and proficiencies
3) The possibility of starting with an ability score of 18 (via an appropriate half feat - most of which arent very good) instead of 2 x 16's for Vuman
If any of those things breaks your game, or is worthy of such handwringing, your game has bigger problems than custom lineage.