Can an 8 speed chain fit inside an 11 speed Shimano 105 (RD-R7000 ) rear derailleur?

The internal width of the chain will be fine, as this has not changed in decades, so it will roll over the cogs and jockey wheels. The problem will be the external width of the chain. It will probably not fit though the 11 Speed derailleur cage, and if it does is likely to rub in some gear combinations. According to Park Tools, 11speed is 5.5mm and 8 speed is 7mm.

On balance it is one of those things that might work, might not, where 'work' is something you define. If it works, mixing and matching components like this is as likely to end up with Claris performance at 105 prices as anything.

I think this is a non-issue. If the 8 speed chain seems slightly too wide for the cage, which is quite likely, you can put a thin washer or spacer on each side of the jockey wheels (as the cage is split in two halves) or use 8sp specific aftermarket jockey wheels with the same number of teeth.