I have a Samsung A21s phone, it’s bootloader is unlocked, Should i install LineageOS (or another custom ROM), or stay in stock Android (Android 12)?
It’s best to change your phone security wise.
with LineageOS assuming there’s even official builds it’s at best a harm reduction but even then the unlocked bootloader is not helping it. Both stock and Lineage are on the same security levels anyways.
One’s is out of date on Android security patches (unless you can get them from Google), the other is the bootloader being permnaetly unlocked, which there is a reason we recommended pixels with GrapheneOS and that’s the capability of locking the bootloader after install.
If your focus is on privacy instead, sure it’s not the worst idea but try to get any official builds first.
First of all, thank you for your reply, I would like to ask if I can reduce the security risk more if I change the device to use LineageOS and add Google Play Service?
no as I said it’s the same security wise.
Then again Lineage while can act as harm reduction, you’re really not getting any better security imo, Would save you on the operating system security but the bootloader being unlocked means with the right vulnerabilities exploited on the system, Someone could very well modify your operating system in a malicious way without you knowing.
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android 15 is objectively more secure than android 12
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yes I’m not doubting Android security here (the OS) but unless they can upgrade to a pixel and use the A12 to perhaps experiment with flashing other operating systems or rooting or etc. Basically without needing to put it into e-waste (and without it being your main phone).
It’s pick your poison situation here. (And it’s about understanding the risks of each one)
I have Samsung S10 running the latest OS/patches from 2023. Would it be safer to install LineageOS on it, unlocking the bootloader in the process, than to keep using the stock OS?
I would check if you have the option to receive security updates from Google, iirc I’ve seen phones that have it. It would be more secure in this case than flashing a ROM.
In the end as I said it’s pick your poison and understanding the risks behind either option.
This is a very small percentage of fixes, I had stats for it here under the Google Play column: Patch Counts - DivestOS Mobile
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mm I see, I guess it’s still pick a poison situation.
Though having both the most possible up to date security update from Google and Bootloader locked perhaps could be better.
the Lineage route is also fine but I would like people to acknowledge the risks behind operating an unlocked bootloader phone around and if possible get a pixel instead, if possible.
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I forgot to point out that I own a Pixel and I was just hoping to use the A21s as a backup device.
Ah this is an important distinction to put.
Then yes Putting an officialy supported operating system like LineageOS if available is not a bad idea for this case.
Granted I would still check if you can receive some form of Google Play security updates on the Samsung phone but if not, Yeah 
If I install Google Play Service on LineageOS, can I get Google Play system upgrades?
not necessary, if official you should receive OTA LineageOS Updates. (recommended to be an officially supported operating system)
I have confirmed that A21s is officially supported by LineageOS, so I don’t need to install Google Play Service to get Google Play system update?
if you need GMS it’s as @anon99163603 pointed or MicroG if you wanna go this route, but you will not need the Google Play System Update as you’ll receive updates from Lineage OTA anyways and they include OS security updates.
If I can get Google Play system update on A21s, would it be safer to stay on Stock OS than switch to LineageOS?
sure but as fellow @SkewedZeppelin said and tracked, it’t not alot and that could also mean it may not be up to date, it’s way better than being 3 years behind security updates or something for sure and keeps your bootloader locked, but it’s something you can decide based on your threat model 
for example, you may want LineageOS to avoid all the samsung spying at the cost of the bootloader being permanently unlocked. (which is more if you have say just surveillance capitalism threat model which would be a common threat model here.)
My A21s was unlocked a long time ago, does this mean that there is no longer a security advantage in using Stock OS.