A person offered an honorarium to speak at a conference in their area of employment speciality (college faculty, for example) may well be considered self-employed for that purpose, since it is in their "usual line of work". They could deduct the expenses against the income and only pay tax on the net after expenses. This is actually fairly common in my line of work and I have personally done this. Doctors, college faculty, and similar professions might have consulting contracts or speaking arrangements outside their usual W-2 employment.
However, getting paid and 1099-ed in a different year than the expense was incurred creates some problems, especially if you only occasionally perform this sort of work. (If one was a regular conference invited speaker, one could deduct 2019 expenses against other 2019 income. Here, the taxpayer does not have other 2019 self-employment income.)
The simplest answer is to forget about the expenses and report the income and pay the tax on the 2020 tax return. The taxpayer could list the income as miscellaneous income (not self-employment) and only pay income tax, not self-employment tax. (Because, even if paid in box 7 as "compensation" for work performed, the taxpayer can argue it was not compensation for work performed because it was a one-off and they are not engaged in an "ongoing trade or business" as a consultant or conference speaker; and thus only subject to income tax and not SE tax.) The taxpayer can't deduct expenses against miscellaneous income because that deduction was eliminated.
If the taxpayer wants to declare it as self-employment income, they can deduct expenses, but will pay income tax AND 15% self-employment tax on the net. This might result in less tax owed. But it gets tricky if the expenses and income occur in different years and the taxpayer has no other 2019 self-employment income. Proper tax advice from an accountant or enrolled agent would be suggested, except that it might not be worth it depending on the amount of the honorarium.
So for (let's imagine) a $1000 speaking fee with $600 in travel expenses, the choice would be report as misc income in 2020 (pay income tax on $1000, easy) or report as SE income (pay income tax plus SE tax on $400, complicated and requires expert advice). That might be $220 of tax versus $148 of taxes.
