Interesting discussion. Odolvlobo has already written about the basic effects of each:
Inflation encourages spending and investing, but discourages saving and lending. Deflation encourages saving and lending, but discourages spending and investing.
But there is a small, but important twist.
Deflation is when
money is appreciating over time. It is however not the same than if your
average salary (for the same work) is appreciating. Even if we have high (e.g. 10%/year) inflation, your hourly wage could grow 12% per year, and then you have a 2% surplus - your "labor" would be worth more each day.
This is often what is tried to achieve by "traditional" central bank measures, i.e. trying to have a low, but positive inflation (most often the goal is 2%), to encourage spending, and at the same time make grow the real wages (i.e. those adjusted to inflation).
You can then buy each time more things with your salary, even if there is inflation.
How does saving and investing fit in these considerations?
Saving is encouraged if a safe, (almost) risk-less way exists to save your money which gives higher returns than inflation. For example, if a savings account gives interests of 3%/year and inflation is 2%/year, then saving is encouraged even if inflation is over zero. Obviously, if the inflation is negative, then you can "save" even simply holding your money. But this is not necessarily good, as this money is taken out of circulation, while money from a saving accounts boosts the balance of the bank, and they can give out more loans.
Obviously, in the last years due to extremely low interest rates saving accounts returns were near zero, so people went desperately searching other, more risky, saving alternatives. This "cheap money policy" benefitted Bitcoin, after all
About investing, it is even more complex. Companies invest in new facilities etc. if there are 1) market opportunities (i.e. if they can get more profit after having invested) and 2) they can afford the interest rates for the loans they need.
If there is deflation, the problem is 1) is unlikely to be met. Market opportunities, at least in the case of "traditional" joint-stock or limited corporations, need economic growth, as that is the most important factor for a share price increase. Deflation unfortunately often decreases growth as it discourages spending (in our current economic system). 2) can happen, if there is a low-interest policy.
So the effect of deflation on investing is ambiguous (always talking about a pure, traditional capitalist economy, where joint-stock corporations are the most important business model).
However, deflation could encourage investing in companies which are not so much dependant on economic growth. For example, a cooperative, in contrast to a joint-stock corporation or a Limited company, would benefit from deflation, as they are not so dependant on a share price increase, and their costs to operate the company would then be lower each time. Cooperatives benefit their members/investors by providing them services or goods, so there may be always a reason to found one and invest in it.
Long term deflation would thus probably lead to incentives to transition from an economy where joint-stock/limited companies are "the norm" to a model close to cooperatives. This has advantages, as cooperatives are known to be very stable and long-lived, but also disadvantages, as their model does not encourage innovation - innovation would have to be encouraged by external means.
(Sorry for the wall of text
)