Winter & pandemic non usage:

[Gnasher]

I assume you mean like some sort of additive.

Nope.  Like much in the motor/bike trade, quality of products has steadily improved both in how they're made and the materials and finishes used.  Up until the mid 70s ish, the advice was when storing a bike over winter or any prolonged period was to drain the tank, tip in about a cup full of engine oil and use it to coat the inside.  This was due to either poor or back then more common no inner tank coatings.         

I'd never think of needing to do any more than fill it to the top to prevent condensation, but then again I'd probably sell before storing for what I'd consider long term.

Filling the tank wont do any harm, save your pocket  and something I used to do, but again hasn't been really necessary for many years.  Improvements in coatings, steel alloys, more and more have plastic or aluminum.  Most rust is either due to small dents that damage the inner coating and/or water introduced when filling.  Oldest bike I own is my XJR now 18yrs old, there's no rust whatsoever in the tank