ALGORITHM=INPLACE, LOCK=NONE, and forcing them to fail loudly

Online DDL is online for some operations and a full table copy for others, and the only way to be sure is to demand the algorithm you expect.

-- fails immediately if it would copy or lock
ALTER TABLE orders ADD INDEX idx_status (status),
  ALGORITHM=INPLACE, LOCK=NONE;

-- ERROR 1846: ALGORITHM=INPLACE is not supported.
--   Reason: Cannot change column type INPLACE.
--   Try ALGORITHM=COPY.

-- ALGORITHM=INSTANT, for the ones that are metadata only
ALTER TABLE orders ADD COLUMN note TEXT, ALGORITHM=INSTANT;

Failing in a second is much better than discovering at minute forty that the statement is copying forty million rows and holding a metadata lock. The error names the reason, which is the fastest documentation available for what each operation costs. INSTANT covers adding a column at the end and a few other cases in 8.0 and is worth trying first, because when it works the statement returns immediately.