I use Eclipse as IDE. Ever since the Eclipse 2020-09 release, I cannot run Eclipse and instead get the following error:

Incompatible JVM - Version 1.8.0_271 of the JVM is not suitable for this product. Version: 11 or greater is required.

However, if I use the latest LTS version (JDK 11), I get warnings:

Build path specifies execution environment JavaSE-1.8. There are no JREs installed in the workspace that are strictly compatible with this environment.
The compiler compliance specified is 1.8 but a JRE 15 is used.

I see that some parts of the JForex SDK code are deprecated in respect to newer java versions (versions greater than 8). This means that I should stick to 8.

However using an outdated Eclipse version (with bugs and security issues) is not an option either.

What should I do? And when will the JForex SDK be updated to comply with Java 11?

This is critical as the Eclipse 2020-09 codebase is compiled against Java 11. Seems you have solved this by installing or defaulting to a v11 or higher JRE.
With your JAVA_HOME variable set to a compatible v11+ JRE, Eclipse 2020-09, 2020-12, etc will work fine.

This is merely a warning stating that the JRE visible to Eclipse JDT (i.e. the JRE/JDK being used to run/compile your code inside eclipse - separate to the JRE that actually started up Eclipse since these are allowed to be different) is not a "perfect match" for the build target version "1.8".

New JREs/JDKs generally have no problem running/compiling previous version binaries and in this case the message is saying you're using a v15 JRE/JDK environment to run code that has been tagged as v1.8.

You can either:

• Ignore this message
• Install a v1.8 JDK (openjdk, oracle, etc) to some folder then go to Window -> Preferences -> Java -> Installed JREs and add a reference to this v1.8 installation.

If you do take the second option, you should also go to Window -> Preferences -> Java -> Installed JREs -> Execution Environments and mark the v1.8 installation as a "perfect match" under the JAVASE-1.8 environment.

Eclipse is quite flexible allowing you to install all versions of the JDK if you really wanted or needed to.
You can control which runtimes are used on a per project basis with: Project -> Properties -> Java Build Path -> Libraries tab -> JRE System Library => Edit and set the project's execution environment.