Mac users who don’t want to or can’t switch to macOS Catalina can still take advantage of the security content of macOS 10.15.3. Apple is offering security updates 2020-001 for macOS Mojave and High Sierra, which patch the same vulnerabilities as the all-new latest version of Catalina. There is a good package: 27 patches in all! Everything is to be recovered from the updates tab of the Mac App Store.
And since you are there, consider repatriating Safari 13.0.5, also available for Mojave and High Sierra (this version of the browser is provided in macOS 10.15.3). This version brings two security fixes.