So...
Appears as if its just EE stopping people on PAYG getting an eSIM, nothing to do with the network.
If you have the means and get get that lovely yellow bit of card and without EE switching it over for you, then you can do a Sim Swap online from a Physical to an eSim on PAYG.
I've just tested it and now have my EE Pay Monthly eSim, EE PAYG eSIM and my Physical O2 Sim.
Only 1 of the eSims can be active, but using the PAYG i have made/received calls and texts and used the data on it too.
And i can switch back and forth between them pretty freely.
** This post comes with no guarantees or assurances. As EE do not Officially support PAYG, your try this at your own risk **