I agree.
Indoors is mostly used as an adverb, and so is not covered by the "plurals" rule. And when it is used as a noun it has no corresponding singular form
indoor. In fact it is likely to be treated as a singular: "indoors is my favourite milieu", rather than "indoors are my favourite milieu". For example, the clothing site
Cafe du Cycliste has a heading "Indoors is the New Outdoors".
So
indoors will be accepted from now on. And let's not forget, in this lockdown epoch, that there is also an
outdoors. That word will also be allowed.