Sony said there will be no games on Neo that are not on PS4. So support for Neo will die with the support of the vanilla PS4.
This is the status quo and that's what I get from it, too.
Yet with PlayStation becoming some kind of "environment" with permeable barriers between the generations, Sony has different possibilities to address this issue in the future:
1. (Forced) full forward compatibility of PS5 games on every PS4 (vanilla and Neo) during the
whole generation
2. (Forced) full forward compatibility of PS5 games on every PS4 at the start of the generation (only first ~3 years), then
. 2a. abandon complete PS4 support (devs are still allowed to support it, but not forced to do it)
. 2b. abandon vanilla PS4 support but keep PS4 Neo support
3. Offer exclusive PS5 games but also enable forward compatibility games should devs want to offer them
4. Offer only unique PS5 games (no FC at all), keep vanilla and (unlikely!) or Neo SKU in portfolio as cheap entry solution to PlayStation environment
I asume Sony will choose the one they deem most effective to ensure a seamless migration of PS4 users to the new one, while at the same time offering an unique incentive / proposition to do so in the first place.