If I block a flyer with a non-flying non-reach card granted reach by having a +1/+1 counter and the card longshot squad on battlefield and then remove the counter with retribution of the ancients before damage is dealt, is that card no longer able to block? Or if the same creature attacks with abzan falconer and then has the counter removed after blockers are declared? Would they be able to redeclare a blocker?
Once blockers are assigned, you do not recheck legality of assignments again. The creature is considered blocked and combat damage is exchanged between them normally. The same would be true if an attacking creature became unblockable after a blocker was assigned to it. It would still be blocked.
From the Comprehensive Rulebook:
509.1g Each chosen creature still controlled by the defending player becomes a blocking creature. Each one is blocking the attacking creatures chosen for it. It remains a blocking creature until it’s removed from combat or the combat phase ends, whichever comes first. See rule 506.4.