Hi Derk,
As you can see in the diagram, 3.1 is an LTS release of Moodle, but even those have an end date for security fixes: May 2019.
Overview of end dates for security fixes:
- Moodle 3.1 LTS ==> May 2019 (first release May 2016) - life time 3 years
- Moodle 3.2 ==> May 2018 (first release November 2016) - life time 1,5 years
- Moodle 3.3 ==> November 2018 (first release May 2017) - lifetime 1,5 years
- Moodle 3.4 ==> May 2019 (first release November 2017) - lifetime 1,5 years
- Moodle 3.5 LTS ==> May 2021 (first planned release May 2018) - life time projected: 3 years or more
I can see why you'd want to use a maintenance friendly version of Moodle, but there may be a challenge ahead, as you might need to upgrade again next year, to 3.5 probably.