Bitcoin Core probably will never have a "1.0". 1.0 is a socially significant number which indicates some kind of finality or completeness (at least initially). This itself is holding back developers from declaring that there may even be a 1.0 or creating some timeline for a 1.0. Bitcoin Core is being continuously developed and there is no point at which many developers foresee it becoming this kind of "final" or "complete" that 1.0 implies.
There is no timeline nor a roadmap for 1.0. There isn't consensus that there should even be a 1.0. There isn't consensus on what features would be required for a 1.0. It is highly unlikely that there will ever be a 1.0. What is more likely to happen is that versioning stays as it is. Maybe in the future the leading 0 will be dropped, or perhaps some other continuous versioning scheme will be used (such as a date based one). But because of the social implications of 1.0, that will probably never happen.
A big part of this has to do with the release cycle of Bitcoin Core. Releases are done in a time based fashion - every 6 or so months, a new release is made. The primary version number just keeps getting bumped for every release. If Bitcoin had started with a 1.0 instead of a 0.1, we'd probably see the same pattern happening today, it would just be 19.1 instead of 0.19.1. But going to a 1.0 is changing the versioning scheme, and there is simply no reason to do that with the given versioning scheme. It would imply some kind of feature change or a major shift in development direction.
From 22.0 onwards, the leading 0 has been dropped from the version number. This was implemented in PR 20223. The last version to contain the leading 0 is 0.21.