Mastodon now has a built-in reminder to add alt text descriptions when posting images, video or audio:
If you're using the website or web app:
1. Go to ⚙️ Preferences (on a phone click ☰ in bottom right corner, then Preferences)
2. Scroll down to Advanced Settings section
3. Tick box marked "Warn me before posting media without alt text"
4. Click "Save Changes"
If you're using an app, this option should be in the app's Settings section (exact wording varies by app).