Which version of ROS2 is recommended for Ubuntu 20.04

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Would this not depend on your own requirements / constraints?

If you're looking for "the most recently released ROS 2 version on 20.04", then you have no choice but to install Galactic.

If you're looking for stability and LTS support, go with Foxy.

If you actually would like to use a newer release, use the Docker images of Humble and you wouldn't be limited by your host OS.

Edit:

Why the Galactic has shorter EOL than Foxy?

Because it's a non-LTS release. From here:

Support

LTS releases

Since Ubuntu LTS releases come with 5 years of standard support we aim to match the same duration. In even years new ROS 2 releases will happen one month after the Ubuntu LTS has been released (which usually means a ROS 2 release in May). The ROS 2 release will be supported until the end of the standard support of the Ubuntu LTS release which is 4 years and 11 months from the ROS 2 release date.

Non-LTS Releases

In order to provide frequent releases to the community, in odd years a non-LTS ROS 2 release will be published. It will always target the same Ubuntu LTS as the previous ROS 2 LTS release but will only be supported for 1.5 years. This duration ensures that the non-LTS will overlap with the next ROS LTS release by 6 months to provide a long enough transition window.

Releases and support duration

• May 2021: G Turtle: non-LTS release, supported for 1.5 years
• May 2022: H Turtle: LTS release, supported for 5 years
• May 2023: I Turtle: non-LTS release, supported for 1.5 years
• May 2024: J Turtle: LTS release, supported for 5 years
• and so on, alternating annually between LTS and non-LTS releases