Is it possible to encrypt system partition with custom rom? I have galaxy s9 and i wanted to have lineageos rom.

So i unlocked bootloader and wiped stock firmware of galaxy s9.And then i installed lineageos 15.1 with twrp 3.2.3-0

Then i encrypted phone. I was thinking that by encryption device in adition to data patition, system partion also being encrypted. now i found that just data partition will encrypt and system partition will not encrypt.

so is there any way to protect system partition? in other words is there any way to encrypt system partition?

sorry  for poor english...

thanks

UPDATE : 

sorry, It is possible that i am wrong or i don't understand what you mean but
I found from link below Boot verification occures when stock rom is installed and and bootloader is locked. but in my case bootloader is unlocked and therefore verifying not occures. 

So i think there is no official way to check integrity of system partition and i should do another things like start a daemon in init.rc for verifying system partition or etc.

Is it true ?

And i should say i put an third-party app in system/app in twrp after mounting system partition and after restart it worked successfully.

Android boot flow
Android does not encrypt system partition as there is no confidential data on the /system. There are only standard ROM files which are already available in public domain. 

The encryption should only apply to the userdata partition /data or SD Card (if mounted as internal storage). Moreover /system is mounted as read-only. So, nothing can be tampered there.