Here are some older versions of the VST SDK:
3.6.12
https://download.steinberg.net/sdk_downloads/vstsdk3612_03_12_2018_build_67.zip
3.6.11
https://download.steinberg.net/sdk_downloads/vstsdk3611_22_10_2018_build_34.zip
3.6.10
https://download.steinberg.net/sdk_downloads/vstsdk3610_11_06_2018_build_37.zip
3.6.9
https://download.steinberg.net/sdk_downloads/vstsdk369_01_03_2018_build_132.zip
3.6.8
https://download.steinberg.net/sdk_downloads/vstsdk368_08_11_2017_build_121.zip
3.6.7
https://www.steinberg.net/sdk_downloads/vstsdk367_03_03_2017_build_352.zip
3.6.6
https://www.steinberg.net/sdk_downloads/vstsdk366_27_06_2016_build_61.zip
3.6.5
https://www.steinberg.net/sdk_downloads/vstsdk365_12_11_2015_build_67.zip
Unfortunately, some of these links don’t seem to work anymore.
EDIT: added some links from the Wayback Machine (thanks to @Golitan11 for the idea).