Spine can always open projects from older versions. Note however that projects saved in newer versions of Spine cannot be opened with older versions.
For exported data, the latest runtimes generally only support data exported from the latest Spine. The runtimes may or may not successfully load data exported from older versions of Spine. If using JSON, it is less likely you'll have to export again. Binary is much less forgiving, if the format changes at all then you'll need to export again.
Both of your projects opened correctly with 1.8.18 and I was able to export and load the data in a runtime (spine-libgdx).
