Copy-pasting from another reply I made:

Summoner icons are always sorted in descending index order. The Ocean Week icons (which includes the lifesaver icon that most people refer to when they complain about this) have indexes in the 2072-2077 range, and the next lowest icon's index at the moment being 1670 (one of the new Yasuo icons). This gap would take maybe until early next year for Riot to fill (a LOT of icon indexes are taken up each year by eSport team icons).

Riot can't just renumber the Ocean Week icons, so the only trivial thing they really can do to fix the issue is to just start adding new icons with indexes starting after the Ocean Week icons. This is exactly what they are doing on PBE for patch 7.13, where there are currently three new team icons as well as six Rift Rivals icons that have been numbered starting immediately after the Ocean Week's icons.