OpenAI now has an official Custom GPT retirement FAQ — Plugins are the recommended replacement

OpenAI has officially documented the retirement of Custom GPTs, recommending Plugins as the replacement, with migration targeted to begin on September 17. For Enterprise workspaces, retirement is planned for December 11. OpenAI notes that GPT instructions can become a skill within a Plugin, but custom actions from connected apps do not automatically migrate. Migrated Plugins may behave differently and require testing against existing workflows, suggesting that this transition might not be a simple, one-click replacement for complex Custom GPT uses.