I noticed the same thing.  I also thought that no exemption was allowed on a final return.  However, when I submitted a final return for an estate in October of 2020, we received an IRS notice because the IRS made an adjustment allowing the exemption and decreasing the estate's taxable income by $600.  I realize that the IRS also makes errors, so please do your own research.  When I reviewed the instructions to Form 1041 a second time, I realized that the instructions do not explicitly say that no deduction is allowed in the final year as many people claim they do. 

The instructions say, "If the estate or trust has for its final year deductions (excluding the charitable deduction and personal exemption) in excess of its gross income, the excess deductions are allowed to the beneficiaries succeeding to the property of the estate or trust..." (Page 27 of 2020 Instructions for Form 1041)  You cannot use the $600 exemption to increase the deductions on termination or to reduce the income distribution deduction.  If the estate has expenses in excess of income in its final year, or if all income is distributed to beneficiaries in the final year, as is usually the case, the common thinking that no exemption is allowed is definitely correct.

However, things are less clear if the estate has taxable income after deducting allowed expenses and the income distribution deduction.  In that scenario, it appears that the $600 exemption is allowed, even in the final year.  Interestingly enough, even though ProSeries doesn't calculate and automatically enter the exemption, it will allow you to enter the $600 exemption without doing an override...at least it did in the 2019 software. 

There may have also been some confusion regarding the suspension of personal exemptions under the TCJA.  The IRS clarified in Proposed Reg 113295-18 (Issued in September of 2020) that the $600 personal exemption is still allowed for an estate even though personal exemptions have been suspended for 1040 purposes.

Again, I caution you to do your own research, but I don't think that this is always as clear cut as some people think it is.