Is it bad to omit semicolon in MySQL queries? [closed]

If the system is able to tell the end of the statement without the semicolon, omitting them does no harm. If the system gets confused, it matters. Since you've been able to leave them off, there isn't apparently a problem. A lot depends on how you're writing your SQL. If you're writing single statements in, say, PHP and then sending them to MySQL for processing, the semicolon is optional.

You ask if it "might have possible negative effects maybe during server high load, caching etc." The answer to that is 'No'. If it has an effect, it is on the basic interpretation of what you meant, and the difference is almost inevitably between 'it works' and 'it does not compile, let alone run'. Effects such as load or caching are completely independent of the presence or absence of semicolons.

This answer applies fairly generally. There's a reference to an SQL Server question which suggests that SQL Server did not need semicolons but is being changed in more recent editions so that they are necessary. It applies to most other DBMS. SQL command interpreters working on an SQL script need to know the boundaries between SQL statements, and the semicolon is the standard way of representing that, though there are other conventions (I believe some use go and some use a slash / for the job). But fundamentally, it comes down to "does it work". If it does, then the semicolon was not necessary.