I created a .gitmodules file in the root of MASTER's project:
[submodule "SLAVE"]
path = SLAVE
url = ../../my-group/SLAVE.git
Added to MASTER's .gitlab-ci.yml:
variables:
GIT_SUBMODULE_STRATEGY: recursive
Triggered MASTER's CI pipeline.
As a result, no changes made in SLAVE project were applied while running MASTER's CI