Then, how likely is it that the format of transactions, generated by the watching version to be signed, will change? Or, is it 'set in stone' and fixed by the Bitcoin protocols?
I think it should work if you generated an unsigned transaction from a watch-only wallet's latest version it will generate a PSBT file you can then import it to your electrum 3.8.8 and be able to sign it. And transfer it back to a new Electrum wallet.
I don't heard yet that it didn't work but if it didn't work you can able to use coinb.in to generate an unsigned transaction and sign it with your old Electrum wallet.