Versions 14 & 15 can only read files back to version 8.1.
They can only write files to the previous version (i.e. 15 can oly write back to 14).
There is a full list of compatibility here...
http://www.archicadwiki.com/ArchiCAD%20versions
Who knows what newer versions will do.
You will also find that old versions will not run on future operating systems.
For example there are issues running version 10 on Windows 7 and there are similar isues with new Macs running old versions.
If a file is important and will be used in the future then it is probably wise to migrate it to new versions as they come along.
Maybe not for every version but certainly when it becomes apparent that an old version will no longer work.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11