2 corollary questions came in last week:

How long does a Company page post last? I can only scroll back about 1 year.

If there are older posts, can anyone still see it?

After a peek in the Help Center (no mention) I suggested they tweet the question to @LinkedInHelp (yes, tweet as in use Twitter to get immediate LinkedIn help).

The answer:

For both company pages and personal profiles, the posts drop off after 1 year or 500 posts, whichever comes first.

To memorialize the best of your posts on either your company Page or personal profile, I suggest using the Featured section. You then control the placement of the post. Think about adaption it for Honors/Awards, Publications, or cite it as part of your work in your About or Experience sections.

The metaphoric snowflake in the snowstorm gets buried.

Just keep it handy for your own later reference. LinkedIn is not a file cabinet.