Every opportunity to have a user open the app is one you should use or make available. I believe having all the users open the app more often (by also making the transfer notifications free) is worth more than having a very small amount of users pay money for said feature (if that's even the difference in buying supporter or not).
I'm no expert in mobile gaming, but as far as I know engagement (user retention, # sessions, daily active users etc) is one of the most important KPI's for mobile games, even before monetization (because engagement improves monetization). Notofications drive engagement (makes users open the app at times they might have not opened it themselves), so you want to make optimal use of them, not have users pay for them.
Your point is valid. I guess most notifications should be made available for all players. But... I also think supporter money also makes for a lot of BB's revenue and you should give supporters that plus (something that is not essential but makes things a bit smoothier). Having 8 skill slots instead of 4 while searching for a player is one of those things.
That said, one kind of notification that should be made available for supporters is when a specific type of player that is defined (let's say a 18yo with 4k+ salary MVP+ potential and 8+JS 8+OD 6+ST 6+FT) is made available on the transfer list. This kinda thing.