Short answer: The ACA ban applies to health coverage, and disability income coverage is an excepted benefit in all circumstances, so it falls outside those rules. Group LTD commonly limits claims for conditions treated shortly before coverage began, for example with a 3/12 clause.
29 CFR 2590.732(c)(2) lists disability income coverage among the benefits excepted in all circumstances, along with accident-only coverage, workers’ comp and liability insurance. The ban on pre-existing condition exclusions therefore does not reach STD or LTD.
Group LTD usually limits pre-existing conditions with two numbers. A 3/12 clause means a condition the employee was treated for, or took medication for, in the 3 months before coverage began is not covered if the disability starts in the first 12 months of coverage. Other look-back and exclusion periods are also used. Many STD plans have no limitation, but some do.
When a plan moves carriers, ask the new carrier for continuity of coverage credit, often called no loss/no gain, so employees who were already covered do not start a new pre-existing clock. Terms vary by carrier.