Markov Crusader

• WOTC Official Reminder Textofficial rules
  Lifelink — Damage dealt by this creature also causes you to gain that much life.

• WWizards of the Coast2016-07-13
  Official Ruling
  If Markov Crusader loses haste before the declare attackers step on the turn it enters the battlefield, it won't be able to attack. Once you've attacked with Markov Crusader, causing it to lose haste won't remove it from combat.

Rulings via Scryfall