Spring Boot 2.3.12.RELEASE was released on 10th Jun 2021 and in July it's been removed from "spring initializr" page (https://start.spring.io/).
My questions:
Is 2.3.x deprecated already?
Yes. Spring Boot 2.3.x was removed from https://start.spring.io as 2.3.x reached the end of its open source support period on 15 May 2021.
If I want to generate a new Spring Boot project for version 2.3.12, how can I do it?
Ideally, you shouldn't. Any existing projects that are using Spring Boot 2.3.x should be updated to 2.4.x or 2.5.x as soon as possible and new projects should use 2.4.x or 2.5.x from the outset.
If you have to start a new project using an unsupported version of Spring Boot, you could generate the project and then modify the version in the pom.xml or build.gradle file to downgrade the version. This may get harder to do the longer the version has been out of support.
If you are using Spring Cloud, you can use start.spring.io's info endpoint to help with version mappings:
…
"spring-cloud": {
"2020.0.0": "Spring Boot >=2.4.0.M4 and <=2.4.0",
"2020.0.0-M3": "Spring Boot >=2.4.0.M1 and <=2.4.0.M1",
"2020.0.0-M4": "Spring Boot >=2.4.0.M2 and <=2.4.0-M3",
"2020.0.3": "Spring Boot >=2.4.1 and <2.5.4-SNAPSHOT",
"2020.0.4-SNAPSHOT": "Spring Boot >=2.5.4-SNAPSHOT",
"Hoxton.SR12": "Spring Boot >=2.2.0.RELEASE and <2.4.0.M1"
},
…
The above indicates that Spring Cloud Hoxton.SR12 should be used with Spring Boot 2.3.x (and 2.2.x).
Note that Spring Cloud Hoxton is also out of its OSS support period so you really should be looking to upgrade to Spring Boot 2.4.x or 2.5.x and Spring Cloud 2020.0.x.