Excluding log shipped DB from current database maintenance plan (Ola's solution)
When you set up Log Shipping, it creates a job for transaction log backup. If you already have an existing transaction log backup job, irrespective of kind (maintenance plan, Ola's solution, SQL Agent job) should be disabled/deleted.
There is a workaround to keep the existing transaction log backup job and set up Log Shipping. You can disable the job that gets created during the Log Shopping setup.
Sample code:
-- Execute the following statements at the Primary to configure Log Shipping 
-- for the database [HostName\SQL2019].[DbaDatabase],
-- The script needs to be run at the Primary in the context of the [msdb] database.  
------------------------------------------------------------------------------------- 
-- Adding the Log Shipping configuration 

-- ****** Begin: Script to be run at Primary: [HostName\SQL2019] ******


DECLARE @LS_BackupJobId AS uniqueidentifier 
DECLARE @LS_PrimaryId   AS uniqueidentifier 
DECLARE @SP_Add_RetCode As int 


EXEC @SP_Add_RetCode = master.dbo.sp_add_log_shipping_primary_database 
        @database = N'DbaDatabase' 
        ,@backup_directory = N'\\logshipping\backup' 
        ,@backup_share = N'\\logshipping\backup' 
        ,@backup_job_name = N'LSBackup_DbaDatabase' 
        ,@backup_retention_period = 4320
        ,@backup_compression = 2
        ,@backup_threshold = 60 
        ,@threshold_alert_enabled = 1
        ,@history_retention_period = 5760 
        ,@backup_job_id = @LS_BackupJobId OUTPUT 
        ,@primary_id = @LS_PrimaryId OUTPUT 
        ,@overwrite = 1 


IF (@@ERROR = 0 AND @SP_Add_RetCode = 0) 
BEGIN 

DECLARE @LS_BackUpScheduleUID   As uniqueidentifier 
DECLARE @LS_BackUpScheduleID    AS int 


EXEC msdb.dbo.sp_add_schedule 
        @schedule_name =N'LSBackupSchedule_HostName\SQL20191' 
        ,@enabled = 1 
        ,@freq_type = 4 
        ,@freq_interval = 1 
        ,@freq_subday_type = 4 
        ,@freq_subday_interval = 15 
        ,@freq_recurrence_factor = 0 
        ,@active_start_date = 20201029 
        ,@active_end_date = 99991231 
        ,@active_start_time = 0 
        ,@active_end_time = 235900 
        ,@schedule_uid = @LS_BackUpScheduleUID OUTPUT 
        ,@schedule_id = @LS_BackUpScheduleID OUTPUT 

EXEC msdb.dbo.sp_attach_schedule 
        @job_id = @LS_BackupJobId 
        ,@schedule_id = @LS_BackUpScheduleID  

EXEC msdb.dbo.sp_update_job 
        @job_id = @LS_BackupJobId 
        ,@enabled = 0 


END 


EXEC master.dbo.sp_add_log_shipping_alert_job 

-- ****** End: Script to be run at Primary: [HostName\SQL2019]  ******