That is a great pic Slowcrash. But, I think it is the "keepers" that would be sunk below that line that is drawn on the "retainer" in your pic. Correct? Normally with an uncracked retainer the keepers are visible with the top of the keeper approximately flush with the line on your pic. From the top the retainers themselves generally look fine, even if they are cracked underneath. But, the unseen cracks in the retainer allows the keepers to sink down lower than normal. Right?

To the tread starter - like the others have said - this is the perfect time to pull your valve cover, inspect your retainers, and check/adjust your valves if needed. Order a gasket set in case you need it for reassembly. Given the time since it was last pulled off, (if ever), I would just replace the valve cover gasket and maybe even the spark plug tube seals. Have you ever adjusted valves before?

I predict your retainers are fine. But, there is only one way to find out for sure. You have to have a look.