If you rank up from one rank to the other when the season ends you get the rank rewards for that rank. Sideswipe will not give you multiple titles for a specific rank only the highest rank you achieved (this situation does not apply to the extra gamemode).
If you rank down out of said rank when the season is over you will still receive the rewards and or titles for the achieved rank, but you will be demoted to Rank 1 of the rank you dropped down to.
Reminder: To obtain the rewards for the refreshed season for example Season 31, you need to play a game in said rank and win it.
When a season is ending and you are Division 0 of the achieved rank, you will automatically be demoted to whatever rank is below said rank. The game does this to help with rank distribution.
(I am not 100% certain this last statement is true however because I, too sometimes stay in the rank I am in Division 0 of.)
Season rewards fluctuate over time being:
Avatar, Banner, Item, or Titles.