Culling, Relationship Culling & Decay: Current Status

@Artaios
Dead played Sims are counted in the 305, they are considered more important to keep than unplayed Sims by the game. Dead unplayed Sims are kept in a minimal state (that's the 100 dead relatives, which aren't counted in the 305).

Keep in mind that the 305 is just a soft cap. You can have more than 305 Sims in your save game (for example with played Sims or housed Sims), though you'll have a higher turnover of townies.