Timber already handled for you where the debug message will not be shown in released apk. If you still want to strip it off, you could. You need to use https://android.googlesource.com/platform/sdk/+/master/files/proguard-android-optimize.txt . But note the warning.
Adding optimization introduces# certain risks, since for example not all optimizations performed by# ProGuard works on all versions of Dalvik. The following flags turn# off various optimizations known to have issues, but the list may not# be complete or up to date. (The "arithmetic" optimization can be# used if you are only targeting Android 2.0 or later.) Make sure you# test thoroughly if you go this route