Hi @Jeroen Bleeker ,
SCCM 1810 is end of life, yes, you are right, it is recommended that we could upgrade the latest SCCM version on the side of server, and the latest SCCM version is applicable for server which is within the support lifecycle, for example, Windows Server 2012 R2.
Here are these articles about End Of Support Dates For SCCM CB and Support for Configuration Manager current branch versions:
https://learn.microsoft.com/en-us/mem/configmgr/core/servers/manage/current-branch-versions-supported
https://www.anoopcnair.com/sccm-life-cycle/
Note: This is non-official Microsoft article just for your reference.
And about upgrading sccm to current branch, please refer to this article:
https://www.prajwaldesai.com/sccm-1906-step-by-step-upgrade-guide/
Note: This is non-official Microsoft article just for your reference.
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