Yeah basically when the PS4 dropped, PS+ was required to play online games against other players on that system. You could still connect to the internet and access features designed for the single player without it, but for anything PvP or co-op you then needed a PS+ subscription. PS3 you could access any online title without an attached subscription or restrictions. I was actually able to play the original Destiny without needing a PS+ subscription on PS4 but I couldn't access PvE/P events - if I tried to join one, it would bring up the Store Homepage.
PS+ on the PS3 from what I remember was very limited and different to what it is now - it was more surrounding exclusivity bonus discounts, free trials of paid games and access to some PS1 classics. I'm not sure if there was any additional features bundled in since the PS4 launched and prior to the PS3 support getting pulled about 4 years ago but it was a very barebones service back in the day from what I remember.
I've managed to dig up an old article with their reasoning for the charge
Sony explains why PS Plus is required for PS4 online play - GameSpot
WWW.GAMESPOT.COM
Shuhei Yoshida says due to "large investment of resources" for online play, keeping service free would have been "absurd."