This is very time-consuming and not worth it, so treat it as educational content.
1.
https://ibotpeaches.github.io/Apktool/install/
2. Take MiuiSystemUI.apk, framework-res.apk, framework-res-ext.apk, miuisystem.apk, miui.apk from your phone and put in in the same folder as 1. These can be found in:
/system/app
/system/priv-app
/system/framework
(You can use ES File Manager or TWRP)
3. Open CMD here and type 'apktool if framework-res.apk -p /your/path/new/Folder' for every mentioned apks except MiuiSystemUI
4. Type 'apktool d MiuiSystemUI.apk -p /your/path/new/Folder'
5. Navigate to MiuiSystemUI\smali\com\android\systemui\statusbar\phone\PhoneStatusBarPolicy.smali and open it with notepad++ or something.
6. Find this code, add the line in blue and save (the
is ';' and ')' autoformatting on forum).
invoke-interface {v3, v4, v0, v1}, Lcom/android/systemui/statusbar/phone/StatusBarIconController;->setIcon(Ljava/lang/String;ILjava/lang/CharSequence
V
iget-object v3, p0, Lcom/android/systemui/statusbar/phone/PhoneStatusBarPolicy;->mIconController:Lcom/android/systemui/statusbar/phone/StatusBarIconController;
iget-object v4, p0, Lcom/android/systemui/statusbar/phone/PhoneStatusBarPolicy;->mSlotBluetooth:Ljava/lang/String;
const/4 v2, 0x0
invoke-interface {v3, v4, v2}, Lcom/android/systemui/statusbar/phone/StatusBarIconController;->setIconVisibility(Ljava/lang/String;Z)V
return-void
.end method
7. In CMD type 'apktool b MiuiSystemUI -p /your/path/new/Folder'
8. Unpack the apk form 'dist' folder by 7-zip.
9. Open archieve, but don't unpack (original MiuiSystemUI.apk) by 7-zip, delete existing classes.dex and drag new one from unpacked from 'dist' folder and close 7-zip.
10. Take this apk from and put in the original path in the phone, make sure it is named 'MiuiSystemUI.apk' (rename original MiuiSystemUI.apk to MiuiSystemUI.apk.old so you can revert in case of problems)
11. Reboot
The Bluetooth icon won't be shown anymore.
If the phone doesn't boot, enter TWRP, mount /system and in file manager delete your apk, and rename the original apk to the original name.
Hope it will work for you.