Personal opinion, don't pay attention to the moderator thingy next to my name.
Yeah, I do think GTA V (SP) modding is declining. I don't think it will completely die, as this is a very popular game, which is decently documented at this point. From looking around scripting peers (though, honestly, I mostly kept to myself with my car/vehicle scripts), people just lose interest. A part has moved on to Red Dead Redemption 2 (newer version of the engine), a part has moved on to FiveM and a small part has gone Patreon-exclusive, with the rest probably moving on with better/more important things in life. You can't expect a mod developer to stay engaged with a single game for years and years on end 
Another part - again, personal opinion - is there being no real "stable" platform for "the" community. Let's look at how scattered things are:
- GTA5-Mods.com - you are here.
- There's the frontpage, and the comments on uploads are pretty active still.
- There is however very little connection with the forums, which I feel are pretty dead aside from support questions.
- There's also a Discord that's impossible to find. Having those three more integrated would probably be nice.
- As for the creator side, yeah, things have dried up, but most things have already matured. Pretty much everything (easily) achievable has been made. I do not think I know of any other game that has such a wide variety of things added by mods. Of course, we don't have gems like Assetto Corsa's Custom Shader Patch, but you can't have everything ¯\_(ツ)_/¯
- I think having a more active general platform/website/community development would have lenghtened the lifetime of it all though, but that's just a feeling.
- GTAForums.com - with GTAGarage, which used to be the place to upload mods and discuss them. Sadly, the V section has completely dried up and no attention has been made to have GTAGarage host GTA V mods. I think most of it has moved to GTA5-Mods.com - with two competing forums, one ends up winning.
- FiveM - Has grown to be massive, with lots of communities and custom mods. The work they did on the game itself is quite amazing, and there's plenty of room to grow. Intermingling with other communities rarely happens though, with most of my interactions being people asking "pls port to fivem server" for my SP scripts...
- RPH/LSPDFR - These guys in their SP police-RP seem to be doing their own (also pretty great) things. The community is rather isolated (from other SP mods), it seems.
- Discord - There are dozens, maybe hundreds, of GTA-related Discord servers. Interaction between them - pretty low, I guess. I'm only in a few, so I wouldn't really know. Also, it's easy to contribute to the problem if you just have a "you" Discord server

- Patreon - With mods like NVE literally taking off
it's awesome for their creators to finally see a net positive from modding, but overall a net negative for modding, as the content and information from especially Patreon (and attached Discord) communities rarely gets shared publicly.
I really think "we" could've done a better job keeping most of it together though, as a few still-active authors have distanced themselves from this website due to disagreements, but also that no "better" platform was offered overall.
Oh well, back to Visual Studio to implement features nobody really cares about 🥲