What a downgrade takes away
In one line: whatever the target version cannot read has to go, otherwise it cannot open the project at all. Losing things is not a side effect — it is the condition that makes downgrading possible.
Removed for certain
Features that did not exist in the target version. If an effect only arrived in 2023 and you bring the project down to 2019, there is no implementation to carry it, so it is removed.
Kept
Sequences, layers, media references, basic transforms and keyframes, plus effects and expressions the target already understands. That is the large majority, which is why a downgraded project usually remains usable.
Judging how much you will lose
The practical way to judge is by what the project does. Cutting and basic grading usually survive well; heavy reliance on the newest effects, automated features or newer formats will show.
Keeping the loss down
Keep the original as your master and treat the downgraded output as a backwards-compatible branch. If you are delivering to someone on an older version, review the downgraded copy first to confirm the key shots are intact.