Microsoft has removed direct links to older SSMS versions from their main download pages, but the files are still hosted on their official CDN. — they can bundle malware.
SSMS 18 was rebuilt using the Visual Studio 2017 Isolated Shell, which brought modern accessibility and security updates. ssms 18 download
– Next week we’ll cover migrating your SSMS 18 registered servers and custom reports to SSMS 20. Microsoft has removed direct links to older SSMS