Calibre, of course, the solution to all your problems.
Thank you very much for your 2 solutions
Well, after testing, the subset of Caliber is much heavier than the subset of FontTool (160 Ko for Caliber versus 8 Ko for FontTool), I would look at the options in detail and the why of the how later but that would be out of subject here.
Here are the command line for those who are interested
Font:
STIX Two Mat
Code range: 2220-222F (∠-∯)
Calibre
Code:
calibre-debug -f "STIX2Math.otf" "STIX2Math-subset.otf" ∠-∯
pyftsubset
Code:
pyftsubset "STIX2Math.otf" --unicodes="2220-222F" --output-file="STIX2Math-subset.otf"