Is there an EOL or a LTS version of Dolibarr
Currently, there is no reason to introduce end of life in a Dolibarr version:
Due to our organization of sources and the good tools, the project is currently able to guarantee that you can install a very old version, use it, and migrate to the last recent one, with no extra cost of work for the project team, even if you wait 10 years before making your migration (Yes, migration from version released 10 years ago is still possible, and this does not cost one penny for that, because Dolibarr is "upgradable by design").
Since now 15 years, Roadmap is clear : There is 1 major version every 6 month (See more information here), so you can amortized your project to deploy an ERP on 10 years if you want, migration project will still be possible and be guaranteed once the version is amortized. This is a very big advantage of Dolibarr software.
A lot of project introduce end of life of version and manage Long Term Support (LTS) version, but this is because they are not able to maintain all past branches.
But the capacity of GIT, of our Continuous Integration system, and of our migration system (Dolibarr is "upgradable by design") allow us to be able to provide a maintenance version (a branch in GIT) for any past released version. So the choice to reduce the version or support to a very limited number of versions depends more on the integrator or the partner that you hire as your maintainer, than on the project itself. Adding restriction and forcing users to upgrade may be a good choice for the the Dolibarr core team if it cost us a lot to maintain old versions, but the truth is that the cost of maintaining old versions is near 0 when you use the correct tools and make the good choices for a software architecture. It seems we made it. So each branch on Github is currently still opened, even if contributions become rare when branch become older.
It is the contributions that the project received (so the real usage on real life) that decides if a branch is still live or not.
For end users, we think this is better than forcing them to upgrades on a system on which they invested a lot to get the perfect processes and needs sometimes 4 or 5 years to be amortized (for large project on medium or very large companies for example, it may be more).
For this reason, there is no official "LTS" version on Dolibarr ERP CRM (or if you prefer, any version is a LTS with an end of life that will be reached when your infrastructure has reached its end of life).
However, there is an important point that you must consider: It is the security.
If for stability or bug fixes, all versions branches are always opened, security fixes are more often submited for recent branches/versions only, so they are not available on too old versions. And it is not possible to backport all security fixes on very old versions (i mean this kind of fixes cost a lot to be backported). So, even if all versions can be considered LTS for user interface, data or stability bugs, it is not the case in a security point of view. For this reason, we recommand to never use a version older than the latest released version of your branch if security is critical (and it should be if your hosting is open on the internet). This is why Dolibarr is "upgradable by design", so you can always upgrade your standard installation of Dolibarr at a low cost, and always benefit of the most secured version.