Can an 8 speed chain fit inside an 11 speed Shimano 105 (RD-R7000 ) rear derailleur? I am using downtube friction shifters, I know my shifter's pull of 28.5 mm multiplied by the Shimano 105 RD's pull ratio of 1.4 gives me 40mm of travel, enough to traverse  the entire width of my 35 mm 8 speed freewheel. I am worried that my 8 speed chain will be too fat to roll across the jockey wheels of the 11 speed 105 rear derailleur...should I be ?
I have an 8 speed freewheel.
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.