Apple has this ugly habit that it does not offer you to download previous versions of its software after newer versions came out. But now with serious Safari 14 problems on Mojave, it might make sense to only upgrade Safari to its latest 13th version.
If you are stuck with Mojave, avoid installing Security Update 2020‑004, Security Update 2020‑005 and Safari 14. These three in combination are causing critical performance issues on Mojave.
Instead, only install Mojave Security Update 2020‑003:
https://support.apple.com/kb/DL2042?locale=en_US
and update Safari to 13.1.2, which is (for now) the last functioning version.
Safari 13.1.2 for High Sierra:
http://swcdn.apple.com/content/downloads/33/34/061-96797-A_PBSVGZX5CC/dt6bdxuqhtv0ck6n0ukx62xry7p6svl9bp/Safari13.1.2HighSierraAuto.pkg
Safari 13.1.2 for Mojave:
http://swcdn.apple.com/content/downloads/38/62/061-98246-A_DETHLIXKPC/njpb63acy0z0rgpdpewxts5rfjlkhtdvcy/Safari13.1.2MojaveAuto.pkg
Safari 13.0 for High Sierra:
http://swcdn.apple.com/content/downloads/56/40/041-98188-A_CVBU5AMWDQ/rca641qzdrg6jscdip42rrsa9860ajps10/Safari13.0HighSierra.pkg
Safari 13.0 for Mojave:
http://swcdn.apple.com/content/downloads/21/46/041-98185-A_T4U6Z3I0PX/tcq0z3b6tmc1xfmz68wfsuhf9dapt5v1u4/Safari13.0MojaveAuto.pkg
Safari 14.0 for Mojave:
http://swcdn.apple.com/content/downloads/20/07/001-50026-A_8ANV58XVC7/nvqtw8plal39y2598inzzco2ekccgrrkx8/Safari14.0MojaveAuto.pkg
Safari 14.0 for Catalina:
http://swcdn.apple.com/content/downloads/00/48/001-50020-A_14M4NE130Q/q4lojf164rqsmp57dawdtgs25ut2d1j44m/Safari14.0CatalinaAuto.pkg