Will he who creates a religious group on a social media application to spread good and remind people of Allah be rewarded, and if innovations or abominations occur in the group after his death, will he be held accountable for them?

Establishing da‘wah groups that remind people of good and help them achieve it is a virtuous act. The one who does so receives the reward of guiding to good, just like the one who performs it. If someone sincerely establishes a group for good, they will, by Allah's will, receive the reward for all the good that is published therein. If the group's state changes after the founder's death, and innovations (bid‘ah) and sins are propagated within it, the founder bears no sin for that. The sin falls upon whoever altered it or introduced the bad practice, as Allah Almighty says: "And no bearer of burdens will bear the burden of another."