This is a known issue - SOTA MT has a billing account with Google, which should cover such eventualities. Up until a few weeks ago, the billing account was held by me, but due to expected high usage of services, and possibly high costs to be accrued by the account, the MT took over the billing for the SMP.
At the moment, the only member of the MT AFAIK with the authority to manage the billing account is Andrew VK3ARR, who is in all likelihood asleep right now, so it may take a little while before he can attend to this. I have also notified Andy MM0FMF by PM, and it may be that he has the requisite knowledge and authority to make the necessary changes in the Google account management site.
This problem has probably arisen because Andrew may not have ticked every box in the account management site.
Basically, Ed, this is an issue which the MT and I have been discussing and moving on for several months, and whether anybody decides to use Google Maps, Leaflet or MapBox as the mapping engine, or Open Maps/OpenTopoMaps/etc. as source for mapping tiles is old news.
EDIT: One mapping site for which I (still) do have a billing account is iotamaps.org, and where I keep a very close eye on page-/map-loads, since it could work out expensive for me if the site gets too popular! And, yes, it’s running fine without any error messages.