Since #32618, getaddressinfo returns iswatchonly: false for every address. It was kept and marked "(DEPRECATED)" so it could be removed later without surprising users (review comment), but it was never put behind -deprecatedrpc. The v30 release notes also say it was already removed from all RPCs, which isn't true for getaddressinfo.
This returns it only with -deprecatedrpc=iswatchonly, like startingheight in getpeerinfo (#34197), so it can be removed in the next release.