How feasible is a remote postdoc in Mathematics?
I’ll only address the US in this answer. Some of this may not be applicable to other countries.
The large majority of US pure math postdocs are mixed teaching and research positions, typically with a three year term. Even though teaching is happening remotely at many schools nowadays, you are asking how departments view the idea of a remote postdoc; well, from a practical point of view, if one is making decisions three years into the future, one cannot assume that teaching is going to continue to be done remotely that far ahead. So, my guess is that you getting such a position as a remote-only postdoc is a non-starter.
That leaves the fairly small number of postdocs that don’t have a teaching component. These come in two flavors. The first is grant-funded and controlled by a specific researcher or research group. These seem to me to be vanishingly rare in pure math these days, but if you are lucky enough to be doing research precisely in an area someone with funding for a postdoc is interested in, you may have a chance. At the very least, it seems more plausible for an individual researcher to be receptive to your suggestion than for a department, committee, fellowship, or other institutional sort of entity.
The other type of positions without teaching are prestigious research postdocs funded by well-known fellowships such as the Miller and Clay. There are only a handful of such fellowships and they are ultra-competitive and extremely difficult to get for anyone, let alone someone who insists on doing a remote postdoc; so getting one of them cannot be thought of as a reliable career plan. But if you’re good enough to be considered seriously for one of those, I can imagine at least a theoretical chance that they might allow you to do the postdoc remotely. You’ll have to check.
The bottom line is that the job market for math postdocs in the US is unfortunately not very friendly towards people in a situation like yours who have geographical location constraints. In any case, good luck!