This is the most searched GTA 6 question after 'how big is the GTA 6 map'. Here's the honest answer, split into three parts, each answered carefully.
See GTA 5 custom maps history for the baseline we're comparing against.
Will GTA 6 console support custom maps?
No. Consoles have never allowed the level of file-system access modding needs. This has been the case for every console GTA, so there's no reason to think GTA 6 will be different.
The official Rockstar Editor and similar options have always had a smaller scope than 'custom maps'. They are tools for creating content, not modding tools.
Will GTA 6 PC support custom maps?
Almost certainly yes - eventually. PC comes out later than console (traditionally 8-18 months). Mods follow the PC release, as per the modding tools timeline.
Single-player custom maps are treated according to Rockstar's official modding policy from 2017 - tolerated, with restrictions applied. Custom maps designed for online play will not be tolerated. Frameworks similar to FiveM may cater to multiplayer custom-map demand.
When realistically will good custom maps exist?
Tier 1: small map mods - less than a year after the PC release. Tier 2: more serious MLOs - less than 18 months. Tier 3: big conversion projects - years away. The big GTA 5 conversions didn't release until 5+ years after the PC release.
If your question is 'will I be able to download a Vice City RP map next year?', then the answer is: maybe a small one. A completed one, no.