Coming from the main page for Jenkins (jenkins.io), selecting ubuntu/debian installation of the LTS version takes you to https://pkg.jenkins.io/debian-stable/
The installation instructions suggest to add deb https://pkg.jenkins.io/debian-stable binary/ to /etc/apt/sources.list, but if you try to install jenkins with that you'll get:
sudo apt-get install jenkins
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
The following NEW packages will be installed:
0 upgraded, 2 newly installed, 0 to remove and 2 not upgraded.
Need to get 68.9 MB/69.0 MB of archives.
After this operation, 70.1 MB of additional disk space will be used.
Do you want to continue? [Y/n] Y
Err:1 https://pkg.jenkins.io/debian-stable binary/ jenkins 2.19.3
Protocol "http" not supported or disabled in libcurl
E: Failed to fetch https://pkg.jenkins.io/debian-stable/binary/jenkins_2.19.3_all.deb Protocol "http" not supported or disabled in libcurl
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
Changing the url from "https" to "http" fixes the problem.
Other installation instructions don't mention the https url: