I am inclined to think this claim can be judged now that BP Statistical review 2019 has been published.
"This claim will be true if before 2020, world oil production declines by at least 3% for 2 years in a row according the figures in the BP Statistical Review of World Energy http://www.bp.com/ (Normally published in June)."
suggests both consecutive declines have to be before 2020.
The last sentence seems to back this up with
"Even if oil declines in 2019 and 2020 but not in 2018 this claim will be judged false."
Only issue is the previous sentence saying
"Note: the last year for which the judge will consider production figures are those for 2020."
Which doesn't seem to agree: why wait for 2020 figures if they do not affect the claim?
I suppose a possible interpretation might be that even though oil production was 2.4% higher in 2018 than 2017, the judge should wait for 2020 figures in case corrections to previous years figures make any difference. If that was the intention, should it have been explained? If not explained is it more likely a drafting error and if so is 2 out of 3 sentences good enough?
The largest decline was 2008 to 2009 with the last report indicating a 2% decline. Adjustment to figures over last 6 years indicate this has varied from 2% to 2.15%. The level of adjustments seems to indicate there is no realistic prospect of revisions to 2018 2.40% increase or 2008/9 decrease changing by as much as needed.
I could see a point in waiting if some years were close to 3% decline but otherwise it seems fairly pointless to wait an extra two years for what looks like an error.