Upgrade script migrates icons to Material Symbols
The SCSS that kept fa-* and oi-* classes working was removed when the web client switched to Material Symbols. The new upgrade_code script rewrites icon usages in XML, JS and other files of custom modules. Run it on your addons before upgrading to 20.0.
Mentioned in: What's new in Odoo 20
Domain condition warnings point to their caller
Warnings about unsupported domain values, such as NewId or records, and about upper-case operators now name the code that built the domain. Run your test suite and outdated patterns in custom modules are easy to find.
Mentioned in: What's new in Odoo 20
Foreign key errors may arrive as RestrictViolation
Depending on the PostgreSQL version, foreign key violations can be raised as RESTRICT violations. The ORM now treats both the same way, and custom code catching only ForeignKeyViolation should do the same.