In patients with low grade gliomas that are older than 40 y/o or have subtotal resections, do you ever withhold upfront RT off protocol?

Yes.
We should be humble about the data supporting RT in this scenario (that is, for IDH-mutant tumors). I would suggest that for IDH wild-type tumors (i.e., molecular GBMs) RCTs in the '70s established an OS benefit for RT and that withholding of RT is not supported.

For IDH-mutant tumors, data from...